A relatively low level of security can be obtained in transporting confidential documents through the mail by cutting the documents lengthwise and mailing the two halves separately. If one half gets misdirected in the mail the unauthorized recipient may find that half somewhat intelligible but, with at least certain documents, will not have truly useful information without the other half.
Numerous optical systems have been proposed through which a document can be copied giving an unintelligible result unless viewed through a similar optical mechanism; see, for example, U.S. Pat. Nos. 3,609,035 and 3,781,109. These devices, of course, require special optical equipment both with the originator and the reader. The single copy contains all the information and if misdirected and unscrambled becomes totally insecure.